The present invention relates generally to brakes for motor vehicles, and more particularly to an apparatus for sensing variation in applied load of a parking brake actuator.
Most vehicle designs incorporate parking brakes. Typical parking brake configurations employ regular drum brakes on a rear wheel with a simple mechanical linkage to engage brake shoes to their respective drums. For instance, an actuator may pull a steel brake cable taut in response to an operator depressing a pedal, lever or button. The resultant tension on the cable is transferred to the end of a brake lever. Other cables may draw the brake shoes firmly against the drums in response to rotation of the lever. A release knob or button causes the actuator to return to its initial position, reintroducing slack into the cables and, consequently, disengaging the brake shoes.
The reliability and integrity of brake performance depends largely upon the manner in which the actuator releases tension in the brake cable. For instance, should the actuator release too much cable, over-release will result. This condition requires the actuator to reel in excessive cable slack prior to a next application, translating into system delays and damage. Conversely, too little release of the cable may result in a brake drag. Such a condition occurs when the brake pads do not fully release. Brake drag retards vehicle performance while causing overheating and damage to brake pads and other system components.
To reduce the occurrence of over-release and drag, some brake manufacturers incorporate systems that use either position or periodic force adjustments to regulate cable release. In particular, one type of position adjustment is made by a position adjuster proximate to the actuator that periodically retracts cable slack to compensate for lining wear and stretching. Another type of position adjustment is made by a position-based release mechanism that consistently moves the actuator a preset distance that ideally corresponds to an optimum brake release point. The preset distance needs to be manually adjusted at service intervals to compensate for system wear factors. In practice, even if serviced regularly, cable stretching and other aging factors cause release points to substantially migrate in between adjustments, allowing the problems of over-release and brake drag. In addition, some users do not service the braking system regularly.
Closed loop position feedback is known in other types of powered mechanisms. In particular, improved position adjustment is achieved with linear position feedback from a sensor (e.g. linear variable displacement transducer (LVDT), linear resistive potentiometers, and optical sensors). However, these sensors are expensive and prone to reliability limitations due to wear and contaminants.
The present invention is an assembly for actuating a parking brake. The assembly comprises a screw, a nut mounted for translation on the screw, and a cable connected between the brake and the nut. A compliant element is mounted on the screw, and is adapted to expand axially with a change in tension in the cable.
Accordingly, it is an object of the present invention to provide a parking brake actuator of the type described above which reliably senses any change in steady state position of the nut.
Another object of the present invention is to provide a parking brake actuator of the type described above which can be electronically operated.
These and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.